1. Field of the Invention
The present invention relates to a thin film transistor. More specifically, the present invention relates to a thin film transistor (hereinafter, "TFT") using an hydrogenated amorphous silicon (hereinafter "a-Si:H").
2. Description of the Prior Art
As is well known, a TFT can be used as a switching element of a pixel of a liquid crystal display. As TFT types, in accordance with positional relationships of an active layer (a semiconductor layer), a gate electrode, a source electrode and a drain electrode, there are a co-planar type, stagger type, and so on. However, a basic structure of such a TFT is as follows: a semiconductor layer is formed on a glass substrate, and a source electrode and a drain electrode are formed on the semiconductor layer, and a gate electrode is further formed on the semiconductor layer separated by an insulation film.
As the semiconductor layer of the TFT, a-Si:H film can be utilized. Such a-Si:H film is normally formed by a plasma CVD method using glow discharge, a direct photo-CVD method using a low pressure mercury lamp, or the like. In a conventional a-Si:H film obtained by such a method, optical bandgap Eopt was 1.7-1.9 eV, and hydrogen content was approximately 20%. In addition, bandtail characteristics energy of the conventional a-Si:H film was more than 50 meV.
If the bandtail characteristics energy is large as in the conventional a-Si:H film, there are many localized states, and therefore, in the TFT utilizing such a-Si H film, a value of average electron mobility .mu. by a field effect is small. Switching speed of the TFT is largely affected by the average electron mobility .mu.. For example, the average electron mobility .mu. in the semiconductor layer of the conventional TFT was approximately 0.5 cm.sup.2 /V sec, and thus, the switching speed was not very rapid. In a case using such a TFT having slow switching speed, i.e. poor responsiveness as a switching element of a pixel of a liquid crystal display, no liquid crystal display with high performance is obtainable.